The present invention relates to a candle precoated filter for residual volume filtration.
More particularly, it relates to a candle type precoated filter for residual volume filtration which includes a cylindrical pressure vessel with a conical base and filter elements which are arranged vertically at collecting tubes and in which at least one residual filter element is lengthened toward the base.
Such candle filters have a cylindrical housing with a conical lower part in which are installed a plurality of filter candles which are generally combined to form registers. The filter candles include a carrier tube on which a rigid or flexible filter medium is arranged. The filtration is effected by flowing through the filter layer from the outside to the inside, a filter cake being formed in a tubular shape on the filter medium. The filtrate flows through the carrier tube to the outlet spout. Cleaning is effected by a brief, percussive reverse flow through the filter medium.
A candle type precoated filter is known from CH-A-658 401 in which one or more filter candles are provided with an enclosing tube which is connected with the filter candle so as to be open at the bottom and sealed at the top. Enclosed and unenclosed filter candles are provided with separate flow-out valves. If the filter inlet and outlet of the unenclosed filter candles are closed and the filter housing is acted upon by a gas, the filter contents are filtered through the enclosed filter candles until the level of the impure liquid has reached the lower edge of the enclosing tube.
However, the known candle filter has disadvantages. For example, when the flow-out of the unenclosed filter candles is closed, the filter cake, depending on its composition, can fall out of the unenclosed filter candles as soon as the filter housing is acted upon by gas due to the absence of differential pressure. The filter cake is then pressed into the narrow annular cross section between the enclosing tube and filter candle and very quickly stops up the latter. This leads to unreasonably long residual filtration times. Even if the filter cake which has dropped down is not pressed into the filter tubes (because the lower edge of the enclosing tubes is arranged quite high in the filter housing), problems occur when cleaning the enclosed filter candles. The filter cake adheres to the inside of the enclosing tube so that the enclosing tube is entirely or partially stopped at least after repeated cleaning.